The invention relates to a power transmission unit having an input section and an output section and a hydraulic clutch which depends on the rotational speed difference, in which unit a hydrostatic displacement machine generates a pressure on the occurrence of a rotational speed difference between the input section and the output section, which pressure causes a reduction in the rotational speed difference, the displacement machine consisting of parts respectively drive-connected to the input section and the output section and one of the sections forming a housing containing the displacement machine, the displacement machine drawing in working fluid from a suction space.
Such power transmission units are more particularly used in drivetrains of motor vehicles, particularly four-wheel drive motor vehicles, either together with a differential gearbox, where the hydraulic clutch limits the differential effect, or for driving the second driven axle, the torque transmitted depending on the difference between wheel rotational speed and the drive shaft connected to the wheels of the other axle. The pressure generated by the displacement machine acts either on a clutch, preferably on a plate clutch, or by hindering the reactive motion of the displacement machine. The effect can be influenced by various valves, either automatically acting valves or externally actuated valves.